Water miscible liquids such as glycols are used in oil and gas production but can become contaminated with dissolved and/or suspended solid matter. Rather than discarding the contaminated liquid, it is generally preferable to remove the solid matter so as to regenerate and reuse the liquid. At many locations worldwide, dissolved salts and other similar contaminating substances are separated from the process liquid (e.g. glycol) by vacuum flash vaporisation processes. Examples of such processes are disclosed in U.S. Pat. Nos. 5,993,608, 6,340,373; 6,508,916, and 6,685,802, incorporated herein by reference, as well as other publications. Industrial plants that apply the flash vaporisation process to glycol are currently in design, under construction, or in operation in the USA, UK, Norway, Brazil, Canada, New Zealand, Australia, India, Russia, Egypt, Azerbaijan, and other countries.
At oil and gas production facilities, the fluids that come from the oil and gas wells may contain substantial amounts of formation water. This, in turn, contains salts and other unwanted substances. At these facilities, mono-ethylene glycol is injected into hydrocarbon flow lines to prevent the formation of hydrates that can plug pipelines. The water then mixes with the glycol to form a dilute aqueous glycol solution. When the crude hydrocarbons are collected at the oil and gas production plant, the dilute aqueous glycol solution is separated from the hydrocarbon fluids. It is then reconcentrated by boiling off excess water, and transported back upstream to be reinjected into the flow lines. In this way, the glycol is reused many times. However, in the absence of treatment, it accumulates unwanted non-volatile solid matter with each recycling round.
The salts and other solid matter can build-up until the level of contamination in the glycol causes increased corrosion, rapid thermal degradation of the glycol, unwanted precipitation of solid matter, fouling of heat transfer equipment and other serious, costly, operational problems. Chlorides, oxides, sulfates, bicarbonates, and carbonates of sodium, potassium, calcium, magnesium, iron, barium, and strontium are examples of inorganic contaminants. Sodium chloride is generally the most prevalent inorganic contaminant. A major source of the salts and other solid matter is the formation water that flows with the hydrocarbon fluids out of the oil and gas production wells. Another source can be the brines and other completion fluids that are injected into the flow lines during or after exploration to prepare for initial production, or as a result of well maintenance activities. Other potential sources include the products of corrosion of the flow lines and the chemicals injected into the flow lines to control corrosion. These non-volatile contaminants must be removed to maintain the quality of the glycol and efficient operations when the glycol is regenerated and reused.
In facilities that treat glycol using a flash vaporisation process, a feed stream comprising an aqueous glycol solution containing contaminants such as dissolved salts is caused to boil rapidly upon mixing with a heated recycle fluid within or in proximity to a flash separation vessel, hereinafter termed the Flash Separator. The vapors that flow out the top of the Flash Separator are depleted of contaminants. Typically, these vapors are either condensed or further separated by distillation into water and concentrated process liquid. The process is normally run under vacuum at an absolute pressure of 0.1 to 0.4 bara so as to reduce the operating temperature when treating a thermally sensitive process liquid such as glycol. Most of the dissolved contaminants, such as sodium chloride, precipitate and fall into a pool of liquid in the lower part of the Flash Separator. The liquid in this pool is a more concentrated solution of the glycol in equilibrium with the vapor phase at about 100 to 150° C. This liquid contains high levels of contaminants in the form of precipitated salt crystals, dissolved inorganic ions and suspended particles. A recycle fluid is drawn from this pool of concentrated glycol in the Flash Separator, heated, and then mixed with the feed stream as described above.
Conventional flash vaporisation process plants typically include additional equipment such as centrifuges, settling tanks or filters to separate the precipitated and suspended solids from the pool of concentrated process liquid in the Flash Separator. The solids are typically then disposed of. These added equipment items have a number of disadvantages such as complexity and high capital cost (centrifuges, filtration), large weight and footprint (settling tanks, filtration), high loss of process liquid with the waste solid matter, and high costs to prevent release of large amounts of process liquid into the environment. These problems are amplified if the flash vaporisation process plant is located on an offshore structure because the use of the added separation equipment described above leads to higher loads on the supporting structure, loss of space, discharge of harmful substances to the ocean, and/or added costs for transport of materials or personnel to/from shore.
In the Flash Separator, sodium, chloride and many other similar dissolved salts precipitate in the form of distinct solid particles, capable of settling, of typically 20 to 100 micron size. However calcium and some other divalent cations are an exception. If the feed stream contains significant quantities of calcium, then in the absence of extra treatment, the calcium accumulates in the recycle fluid. Dissolved calcium, if present, does not precipitate to form well behaved particles in the concentrated glycol. Instead, it combines with glycol and chloride to form calcium-glycol-chloride complex compounds that raise the viscosity of the recycle fluid and solidify if allowed to cool to less than about 100 to 120° C. Over time, as more calcium builds up, the recycle fluid viscosity can become unmanageable. Then, upon cooling, the entire mass of liquid in the Flash Separator can turn solid. This has been a costly experience at several operating plants. Other divalent cations can also cause a similar effect. However, most research and plant design work has focused on finding a solution for calcium, as it is typically the most prevalent divalent cation. The presence of calcium in formation water is not surprising given that it is a major component of limestone and other subsurface rock found in some oil and gas fields.
Plant designers have sought to address the calcium problem by including an additional treatment procedure. This treatment starts by mixing a carbonate containing material, such as an aqueous solution of sodium carbonate, with the incoming glycol upstream of the flash vaporisation process. The calcium ions combine with the added carbonate to form insoluble calcium carbonate which is then mechanically removed in clarifiers and/or filters. This solution has been in use since at least 1994. There are several major drawbacks with this treatment procedure, including: the cost and complexity of adding carbonate to the glycol; the size, cost and complexity of the carbonate filtration equipment; and the high glycol content in the waste solid material. As before, these problems are amplified if the equipment used in the calcium treatment procedure described above is located on an offshore structure because this leads to; higher loads on the supporting structure, loss of space, discharge of harmful substances to the ocean, and/or added costs for transport of materials or personnel to/from shore.
Other less common contaminants have been known to, or have been identified by researchers as having the potential to, cause problems with the flash vaporisation process when used to treat glycol. Three examples are acetate, nitrate and phosphate which can dissolve in glycol and cause undesirable changes to the properties of the recycle fluid.
Thus, there is a need in the art for an improved method for treating water miscible process liquids, for example, fluids used for oil and gas processing, that have become contaminated with dissolved and/or suspended solid matter.